Method and apparatus for header compression

ABSTRACT

A packet compressor  12  operates under a reliable mode or an optimistic mode. A mode determination unit  31  counts the number of ACK packets or NACK packets received by a unit time X by an ACK/NACK packet receiver  14 . When the counted number of NACK packets is larger than a predetermined value Y, the mode determination unit  31  switches the operation mode of the packet compressor  12  to the reliable mode. When the counted number of ACK packets is larger than a predetermined value Z, the mode determination unit  31  switches the operation mode of the packet compressor  12  to the optimistic mode.

This is a Divisional Application of U.S. application Ser. No.09/929,344, filed Aug. 15, 2001 now U.S. Pat. No. 6,889,261.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods and apparatuses for datacompression and, more specifically, to a method and apparatus for headercompression/decompression used in a data transmission system where apacket is compressed in a transmitting side and decompressed in areceiving side.

2. Description of the Background Art

Typical protocols recently known for data transmission over the Internetinclude TCP/IP (Transmission Control Protocol/Internet Protocol) andUDP/IP (User Datagram Protocol/Internet Protocol). One protocol knownfor real-time transmission of data such as audio and images under TCP/IPor UDP/IP is RTP (Realtime Transport Protocol) , which is described indetail in “RTP: A Transport Protocol for Real-Time Applications”, H.Schulzrinne, S. Casner, R. Frederik, and V. Jacobson, RFC1889, 1996.

When data is transmitted by unit of a packet in compliance with theprotocols such as TCP/IP, the transmitting side adds a header to eachpacket. In data transmission over a low and midrange bit ratetransmission path, however, the added header specified by TCP, UDP, IP,or other protocols increases the packet size, thereby disadvantageouslycausing overhead associated with communication. For example, to transmit10-byte data under UDP/IP, the transmitting side has to add a 28-byteheader to the original data, resultingly forming a 38-byte packet, whichis approximately four times larger in size than the original data. Ifsuch increase happens quite often, the effective speed of thetransmission path is substantially decreased.

In order to reduce communication overhead caused by the added header, aheader compression scheme developed by V. Jacobson and defined inRFC1144 and RFC2508 has been known. In this scheme, among the fields ofthe header included in the packet, transmitted are only any fieldchanged in value from the one included in the previous packet. Suchfield changed in value are not so many in the header, and therefore, inthis scheme, header compression poses no special problem.

The Jacobson's header compression scheme is a standard for wiredcommunication with a low transmission error rate, and is not suitablefor wireless communication with a high transmission error rate. Forexample, consider a case where point-to-point communication is carriedout under a protocol such as PPP (Point to Point Protocol) over anetwork shown in FIG. 6A or 6B between a gateway server 72 and aterminal 74 or a portable wireless terminal 77. As shown in FIG. 6A,when the gateway server 72 and the terminal 74 are connected to eachother via a wired communication network 73 constructed by a modem, ISDN,or LAN, the Jacobson's header compression scheme works quiteeffectively. On the other hand, as shown in FIG. 6B, the gateway server72 and the portable wireless terminal 77 are coupled to each other via acellular phone network 75 by W-CDMA (Wideband Code Division MultipleAccess), for example. Such network structure as shown in FIG. 6B hasbeen widely adopted, with a recent increase in cellular phone users. Inthis network, however, transmission errors are prone to occur in awireless transmission section 76, and therefore the Jacobson's headercompression scheme does not effectively work.

One example of the header compression scheme for wireless communicationwhere transmission errors frequently occur is known as ROHC (RobustHeader Compression) studied by IETF (Internet Engineering Task Force).The detail of ROHC is described in “draft-ietf-rohc-rtp-00.txt 29 Jun.2000”.

FIG. 7 is a block diagram showing the structure of a data transmittingapparatus and a data receiving apparatus using ROHC. In FIG. 7, a datatransmitting apparatus 80 compresses the header of each packet byreferring to reference information stored in a reference informationmanager 85, and transmits the compressed packet. A data receivingapparatus 90 decompresses the header of each received packet byreferring to reference information stored in a reference informationmanager 95.

Here, the reference information is information indicating how each fieldincluded in the header of the packet is changed from the one included inthe header of the previous packet. For example, the header includes aUDP port number, an RTP sequence number, and an RTP timestamp. Assumeherein that, compared with the header of the previous packet, the UDPport number is not changed, the RTP sequence number is increased by 1,and the RTP timestamp is increased by 50. In this case, the referenceinformation indicates the states of the UDP port number, the RTPsequence number, and the RTP timestamp as assumed above.

FIGS. 8 to 11 are sequence charts of ROHC. In FIG. 8, the datatransmitting apparatus 80 and the data receiving apparatus 90 both holdthe same reference information α. When transmitting a header H1 and dataD1, the data transmitting apparatus 80 refers to the referenceinformation α stored in the reference information manager 85 to find acompressed header P1 by the following equation (1), and transmits apacket composed of the compressed header P1 and the data D1.P1=H1*α  (1)

where * represents an operation applied to the header and the referenceinformation. The operation * varies for each field of the header.

On receiving the packet composed of the compressed header P1 and thedata D1, the data receiving apparatus 90 refers to the referenceinformation α stored in the reference information manager 95, andcarries out an inverse operation of the operation represented by theabove equation (1) to decompress the compressed header P1 to the headerH1.

Similarly, the data transmitting apparatus 80 then transmits a packetcomposed of a compressed header Pi and data Di, and the data receivingapparatus 90 decompresses the compressed header Pi included in thereceived packet to the original header Hi.

Also in data transmission under ROHC, the manner of changes in the fieldincluded in the header may be varied during data transmission. Forexample, the RTP timestamp has been increased so far by 50, but by 100at some point in time and thereafter. In this case, the datatransmitting apparatus 80 and the data receiving apparatus 90 both haveto correctly update the respective reference information.

The data transmitting apparatus 80 detects that the manner of changes inthe field of the packet to be transmitted becomes varied and, based onthe detection, updates the reference information stored in the referenceinformation manager 85. In the above example, the data transmittingapparatus 80 updates the reference information α from “the RTP timestampis increased by 50” to “the RTP timestamp is increased by 100”. Theupdated reference information is hereinafter referred to as referenceinformation β.

The data receiving apparatus 90 also has to update its own referenceinformation to correctly decompress the header even after the datatransmitting apparatus 80 updates its own reference information.Therefore, the data transmitting apparatus 80 transmits a packettogether with information for updating the reference information at thereceiving side (such information is hereinafter referred to as “updateinformation”). With the received update information, the data receivingapparatus 90 updates the reference information stored in the referenceinformation manager 95, and notifies the data transmitting apparatus 80that the reference information has been correctly updated. The updateinformation may be the updated reference information itself, orinformation indicating the difference between the reference informationα and the reference information β.

In order for the data transmitting apparatus 80 to confirm that thereference information of the data receiving apparatus 90 has beencorrectly updated, two modes are known, that is, a “reliable mode” thatensures the reliability and an “optimistic mode”, in consideration ofcompression efficiency. These modes are described in the above document(“draft-ietf-rohc-rtp-00.txt 29 Jun. 2000”). FIG. 9 is a sequence chartfor the reliable mode. The data transmitting apparatus 80 operating inthe reliable mode updates the reference information from α to β, andthen transmits a packet with the update information added thereto untilreceiving a packet indicating that the reference information of the datareceiving apparatus 90 has been correctly updated. Such packet ishereinafter referred to as “ACK packet”. At this time, the datatransmitting apparatus 80 refers to the updated reference information βto compress the header of the packet with the update information firstadded thereto and the headers of the packets coming thereafter. In FIG.9, the updated reference information β is used as the update informationPi, represents a header compressed by referring to the referenceinformation α, and Qi represents a header compressed by referring to theupdated reference information β.

FIGS. 10 and 11 are sequence charts for the optimistic mode. The datatransmitting apparatus 80 operating in the optimistic mode updates thereference information from α to β, and then adds the update informationto only one packet for transmission. Thereafter, assuming that thereference information of the data receiving apparatus 90 has beencorrectly updated even without receiving the ACK packet, the datatransmitting apparatus 80 transmits packets without the updateinformation added thereto. At this time, the data transmitting apparatus80 refers to the updated reference information β to compress the headerof the packet with the update information added thereto and the headersof the packets coming thereafter. In this case, as shown in FIG. 10, ifthe data receiving apparatus 90 correctly receives the updateinformation and updates the reference information from α to β byreferring to the update information, no problem occurs in datatransmission thereafter.

However, if failing to receive the update information as shown in FIG.11, the data receiving apparatus 90 continues to decompress the headerby referring to the original reference information α. The data receivingapparatus 90 carries out CRC (Cyclic Redundancy Check), for example, todetect any header decompression error. If detecting any headerdecompression error, the data receiving apparatus 90 transmits a packetfor requesting transmission of the update information. Such packet ishereinafter referred to as “NACK packet”. On receiving the NACK packet,the data transmitting apparatus 80 adds again the update information toone packet for transmission. Therefore, the data receiving apparatus 90cannot correctly decompress the header from the time when failing toreceive the packet with the update information added thereto to the timewhen receiving another packet with the update information added thereto.

Described below are the characteristics and drawbacks of the abovedescribed two modes for header compression, that is, the reliable modeand the optimistic mode.

In the reliable mode, the reference information is always correctlyupdated at the transmitting and receiving sides, thereby suppressing theoccurrence of header decompression error at the receiving side. However,the transmitting side continues to transmit the packets with the updateinformation added thereto until receiving the ACK packet, therebydegrading header compression efficiency and entire data transmissionefficiency.

In the optimistic mode, the transmitting side adds the updateinformation to only one packet for transmission and, even withoutreceiving the ACK packet, compresses the headers of the packets comingthereafter by referring to the updated reference information. Therefore,if the data receiving apparatus 90 has correctly received the updateinformation, high header compression efficiency can be achieved.However, if the data receiving apparatus 90 has not correctly receivedthe update information due to transmission error, header decompressionerrors continuously occur until receiving the update information,thereby considerably degrading the data transmission efficiency.

To solve the above problem, the header compression scheme is switchedbetween the reliable mode and the optimistic mode, thereby improving theheader compression efficiency and the data transmission efficiency.However, how to switch the header compression scheme has been unclear sofar.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to present how toswitch the header compression scheme between the reliable mode and theoptimistic mode, and provide a method and apparatus for headercompression/decompression for improving the header compressionefficiency and the data transmission efficiency during wirelesstransmission.

The present invention has the following features to attain the objectabove.

A first aspect of the present invention is directed to a headercompression apparatus for compressing a header of a packet to betransmitted by referring to reference information that is also includedin a receiving side, and the apparatus includes: a reference informationmanager for storing and managing the reference information; a packetcompressor for compressing the header of the packet in a specifiedoperation mode by referring to the reference information, andselectively adding, to the compressed packet, update information forupdating the reference information at the receiving side; a packettransmitter for transmitting the packet compressed by the packetcompressor; a packet receiver for receiving an ACK packet indicatingthat the reference information at the receiving side has been correctlyupdated or a NACK packet for requesting transmission of the updateinformation due to a header decompression error that occurred at thereceiving side; and a mode determination unit for switching theoperation mode of the packet compressor to a reliable mode where, afterthe reference information of the header compression apparatus isupdated, the packet compressor continuously adds the update informationuntil the ACK packet is received, and to an optimistic mode where thepacket compressor adds the update information when the referenceinformation of the header compression apparatus is updated and wheneverreceiving the NACK packet. When the operation mode is the optimisticmode and the number of NACK packets received by a unit time X is largerthan a predetermined value Y, the mode determination unit switches theoperation mode to the reliable mode, and when the operation mode is thereliable mode and the number of ACK packets received by the unit time Xis larger than a predetermined value Z, the mode determination unitswitches the operation mode to the optimistic mode.

According to the first aspect, the transmitting side determines thetransmission quality based on the state of update of the referenceinformation at the receiving side. When the transmission quality isdegraded, the transmitting side switches the header compression schemeto the reliable mode. When the transmission quality is improved, thetransmitting side switches the header compression scheme to theoptimistic mode. As such, by switching the header compression scheme atthe transmitting side, the header compression efficiency and thetransmission quality during wireless transmission can be improved. Inother words, it is possible to reduce time and cost required fortransmission of a packet that cannot be decompressed, and also improvecompression efficiency and greatly reduce cost required for packettransmission.

In this case, the mode determination unit may calculate a rate of changein the number of NACK packets or ACK packets received by the unit timeX, and increase the unit time X when the rate of change is smaller thana predetermined value A, and decrease the unit time X when the rate ofchange is larger than a predetermined value B.

Thus, when determining that the transmission quality is stable sinceNACK packets or ACK packets are received with roughly regular frequency,the transmitting side extends the time interval between possible changesof the header compression scheme and, otherwise, shortens the timeinternal. As such, highly responsive to the change in transmissionquality during wireless transmission, the transmitting side switches theheader compression scheme, thereby improving the compression efficiencyand the transmission quality during wireless transmission. In otherwords, header decompression errors are suppressed for high compressionefficiency, and therefore transmission cost can be reduced.

A second aspect of the present invention is directed to a headerdecompression apparatus for decompressing a header of a received packetby referring to reference information that is also included in atransmitting side, and the apparatus includes: a reference informationmanager for storing and managing the reference information; a packetreceiver for receiving the packet with update information selectivelyadded thereto for updating the reference information; a packetdecompressor, provided with the packet received by the packet receiver,for updating the update information by using the update information anddecompressing the header of the packet by referring to the referenceinformation; a packet transmitter for transmitting an ACK packetindicating that the reference information has been correctly updated ora NACK packet for requesting transmission of the update information whena header decompression error occurs in the packet decompressor; modedetermination unit for switching the operation mode of the transmittingside to a reliable mode where, after updating the reference informationof the transmitting side, the transmitting side continuously adds theupdate information until receiving the ACK packet, and to an optimisticmode where the transmitting side adds the update information when thereference information of the transmitting side is updated and wheneverreceiving the NACK packet; and a mode notification unit for notifyingthe transmitting side of the operation mode selected by the modedetermination unit. The mode determination unit counts the number ofheader decompression errors that occurred by a unit time X in the packetdecompressor. When the operation mode is the optimistic mode and thenumber is larger than a predetermined value Y, the mode determinationunit switches the operation mode to the reliable mode. When theoperation mode is the reliable mode and the number is smaller than apredetermined value Z, the mode determination unit switches theoperation mode to the optimistic mode.

According to the second aspect, the receiving side determines thetransmission quality based on the state of occurrence of headerdecompression errors. When the transmission quality is degraded, thereceiving side switches the header compression scheme at thetransmitting side to the reliable mode. When the transmission quality isimproved, the receiving side switches the header compression scheme tothe optimistic mode. As such, by switching the header compression schemeat the receiving side, the header compression efficiency and thetransmission quality during wireless transmission can be improved. Inother words, it is possible to reduce time and cost required fortransmission of a packet that cannot be decompressed, and also improvecompression efficiency and greatly reduce cost required for packettransmission.

In this case, the mode determination unit may calculate a rate of changein the number by the unit time X, and increase the unit time X when therate of change is smaller than a predetermined value A, and decrease theunit time X when the rate of change is larger than a predetermined valueB.

Thus, when determining that the transmission quality is stable sinceheader decompression errors occur with roughly regular frequency, thereceiving side extends the time interval between possible changes of theheader compression scheme at the transmitting side and, otherwise,shortens the time internal. As such, highly responsive to the change intransmission quality during wireless transmission, the receiving sideswitches the header compression scheme of the transmitting side, therebyimproving the compression efficiency and the transmission quality duringwireless transmission. In other words, header decompression errors aresuppressed for high compression efficiency, and therefore transmissioncost can be reduced.

A third aspect of the present invention is directed to a headercompression apparatus for compressing a header of a packet to betransmitted by referring to reference information that is also includedin a receiving side, and the apparatus includes: a reference informationmanager for storing and managing the reference information; a packetcompressor for compressing the header of the packet in a specifiedoperation mode by referring to the reference information, andselectively adding, to the compressed packet, update information forupdating the reference information at the receiving side; a packettransmitter for transmitting the packet compressed by the packetcompressor; a packet receiver for receiving an ACK packet indicatingthat the reference information at the receiving side has been correctlyupdated or a NACK packet for requesting transmission of the updateinformation due to a header decompression error that occurred at thereceiving side; a delay time measuring unit for measuring a roundtripdelay time with respect to the receiving side by transmitting andreceiving a packet to and from the receiving side; and a modedetermination unit for switching the operation mode of the packetcompressor to a reliable mode where, after the reference information ofthe header compression apparatus is updated, the packet compressorcontinuously adds the update information until the ACK packet isreceived, and to an optimistic mode where the packet compressor adds theupdate information when updating the reference information of thetransmitting side and whenever receiving the NACK packet. The modedetermination unit receives, by a unit time X, the roundtrip delay timefrom the delay time measuring unit, and switches the operation mode tothe reliable mode when the operation mode is the optimistic mode and theroundtrip delay time is smaller than a predetermined value Y, and to theoptimistic mode when the operation mode is the reliable mode and theroundtrip delay time is larger than a predetermined value Z.

According to the third aspect, based on the roundtrip delay time withrespect to the receiving side, the transmitting side switches the headercompression scheme to the reliable mode when the roundtrip delay time isshort, and to the optimistic mode when long. As such, by switching theheader compression scheme by the transmitting side, the headercompression efficiency and the transmission quality during wirelesstransmission can be improved. In other words, it is possible to reducetime and cost required for transmission of a packet that cannot bedecompressed, and also improve compression efficiency and greatly reducecost required for packet transmission.

In this case, the mode determination unit may calculate a rate of changein the roundtrip delay time, and increase the unit time X when the rateof change is smaller than a predetermined value A, and decrease the unittime X when the rate of change is larger than a predetermined value B.

Thus, when determining that the transmission quality is stable since theroundtrip delay time is roughly stable, the transmitting side extendsthe time interval between possible changes of the header compressionscheme and, otherwise, shortens the time internal. As such, highlyresponsive to the change in transmission quality during wirelesstransmission, the transmitting side switches the header compressionscheme, thereby improving the compression efficiency and thetransmission quality during wireless transmission. In other words,header decompression errors are suppressed for high compressionefficiency, and therefore transmission cost can be reduced.

A fourth aspect of the present invention is directed to a headerdecompression apparatus for decompressing a header of a received packetby referring to reference information that is also included in atransmitting side, and the apparatus includes: a reference informationmanager for storing and managing the reference information; a packetreceiver for receiving the packet with update information selectivelyadded thereto for updating the reference information; a packetdecompressor for receiving the packet received by the packet receiverand updating the reference information by using the update information,and decompressing the header of the packet by referring to the referenceinformation; a packet transmitter for transmitting an ACK packetindicating that the reference information has been correctly updated ora NACK packet for requesting transmission of the update information whena header decompression error occurs in the packet decompressor; a delaytime measuring unit for measuring a roundtrip delay time with respect tothe transmitting side by transmitting and receiving a packet to and fromthe transmitting side; a mode determination unit for switching theoperation mode of the transmitting side to a reliable mode where, afterupdating the reference information of the transmitting side, thetransmitting side continuously adds the update information untilreceiving the ACK packet, and to an optimistic mode where thetransmitting side adds the update information when the referenceinformation of the transmitting side is updated and whenever receivingthe NACK packet; and a mode notification unit for notifying thetransmitting side of the operation mode selected by the modedetermination unit. The mode determination unit receives, by a unit timeX, the roundtrip delay time from the delay time measuring unit, andswitches the operation mode to the reliable mode when the operation modeis the optimistic mode and the roundtrip delay time is smaller than apredetermined value Y, and to the optimistic mode when the operationmode is the reliable mode and the roundtrip delay time is larger than apredetermined value Z.

According to the fourth aspect, based on the roundtrip delay time withrespect to the transmitting side, the receiving side switches the headercompression scheme to the reliable mode when the roundtrip delay time isshort, and to the optimistic mode when long. As such, by switching theheader compression scheme by the receiving side, the header compressionefficiency and the transmission quality during wireless transmission canbe improved In other words, it is possible to reduce time and costrequired for transmission of a packet that cannot be decompressed, andalso improve compression efficiency and greatly reduce cost required forpacket transmission.

In this case, the mode determination unit may calculate a rate of changein the roundtrip delay time measured by the unit time X, and increasethe unit time X when the rate of change is smaller than a predeterminedvalue A, and decrease the unit time X when the rate of change is largerthan a predetermined value B.

Thus, when determining that the transmission quality is stable since theroundtrip delay time is roughly stable, the receiving side extends thetime interval between possible changes of the header compression schemeand, otherwise, shortens the time internal. As such, highly responsiveto the change in transmission quality during wireless transmission, thereceiving side switches the header compression scheme of thetransmitting side, thereby improving the compression efficiency and thetransmission quality during wireless transmission. In other words,header decompression errors are suppressed for high compressionefficiency, and therefore transmission cost can be reduced.

A fifth aspect of the present invention is directed to a headercompression method of compressing a header of a packet to be transmittedby referring to reference information that is also included in areceiving side, and the method includes: a packet compression step ofcompressing the header of the packet in a specified operation mode byreferring to the reference information stored, and selectively adding,to the compressed packet, update information for updating the referenceinformation at the receiving side; a packet transmission step oftransmitting the packet compressed in the packet compression step; apacket receiving step of receiving an ACK packet indicating that thereference information at the receiving side has been correctly updatedor a NACK packet for requesting transmission of the update informationdue to a header decompression error that occurred at the receiving side;and a mode determination step of switching the operation mode of thepacket compression step to a reliable mode where, after the referenceinformation is updated, the update information is continuously addeduntil the ACK packet is received, and an optimistic mode where theupdate information is added when the reference information is updatedand whenever the NACK packet is received. In the mode determinationstep, when the operation mode is the optimistic mode and the number ofNACK packets received by a unit time X is larger than a predeterminedvalue Y, the operation mode is switched to the reliable mode, and whenthe operation mode is the reliable mode and the number of ACK packetsreceived by the unit time X is larger than a predetermined value Z, theoperation mode is switched to the optimistic mode.

According to the fifth aspect, the transmitting side determines thetransmission quality based on the state of update of the referenceinformation at the receiving side. When the transmission quality isdegraded, the transmitting side switches the header compression schemeto the reliable mode. When the transmission quality is improved, thetransmitting side switches the header compression scheme to theoptimistic mode. As such, by switching the header compression scheme atthe transmitting side, the header compression efficiency and thetransmission quality during wireless transmission can be improved. Inother words, it is possible to reduce time and cost required fortransmission of a packet that cannot be decompressed, and also improvecompression efficiency and greatly reduce cost required for packettransmission.

In this case, in the mode determination step, a rate of change in thenumber of NACK packets or ACK packets received by the unit time X iscalculated, and the unit time X is increased when the rate of change issmaller than a predetermined value A, and decreased when the rate ofchange is larger than a predetermined value B.

Thus, when determining that the transmission quality is stable sinceNACK packets or ACK packets are received with roughly regular frequency,the transmitting side extends the time interval between possible changesof the header compression scheme and, otherwise, shortens the timeinternal. As such, highly responsive to the change in transmissionquality during wireless transmission, the transmitting side switches theheader compression scheme, thereby improving the compression efficiencyand the transmission quality during wireless transmission. In otherwords, header decompression errors are suppressed for high compressionefficiency, and therefore transmission cost can be reduced.

A sixth aspect of the present invention is directed to a headerdecompression method of decompressing a header of a received packet byreferring to reference information that is also included in atransmitting side, and the method includes: a packet receiving step ofreceiving the packet with update information selectively added theretofor updating the reference information stored; a packet decompressionstep, provided with the packet received in the packet receiving step, ofupdating the update information by using the update information, anddecompressing the header of the packet by referring to the referenceinformation; a packet transmission step of transmitting an ACK packetindicating that the reference information has been correctly updated ora NACK packet for requesting transmission of the update information whena header decompression error occurs in the packet decompression step; amode determination step of switching the operation mode of thetransmitting side to a reliable mode where, after updating the referenceinformation of the transmitting side, the transmitting side continuouslyadds the update information until receiving the ACK packet, and to anoptimistic mode where the transmitting side adds the update informationwhen the reference information of the transmitting side is updated andwhenever receiving the NACK packet; and a mode notification step ofnotifying the transmitting side of the operation mode selected in themode determination step. In the mode determination step, the number ofheader decompression errors that occurred by a unit time X in the packetdecompression step is counted, and when the operation mode is theoptimistic mode and the number is larger than a predetermined value Y,the operation mode is switched to the reliable mode, and when theoperation mode is the reliable mode and the number is smaller than apredetermined value Z, the operation mode is switched to the optimisticmode.

According to the sixth aspect, the receiving side determines thetransmission quality based on the state of occurrence of headerdecompression errors. When the transmission quality is degraded, thereceiving side switches the header compression scheme at thetransmitting side to the reliable mode. When the transmission quality isimproved, the receiving side switches the header compression scheme tothe optimistic mode. As such, by switching the header compression schemeat the receiving side, the header compression efficiency and thetransmission quality during wireless transmission can be improved. Inother words, it is possible to reduce time and cost required fortransmission of a packet that cannot be decompressed, and also improvecompression efficiency and greatly reduce cost required for packettransmission.

In this case, in the mode determination step, a rate of change in thenumber by the unit time X is calculated, and the unit time X isincreased when the rate of change is smaller than a predetermined valueA, and decreased when the rate of change is larger than a predeterminedvalue B.

Thus, when determining that the transmission quality is stable sinceheader decompression errors occur with roughly regular frequency, thereceiving side extends the time interval between possible changes of theheader compression scheme at the transmitting side and, otherwise,shortens the time internal. As such, highly responsive to the change intransmission quality during wireless transmission, the receiving sideswitches the header compression scheme of the transmitting side, therebyimproving the compression efficiency and the transmission quality duringwireless transmission. In other words, header decompression errors aresuppressed for high compression efficiency, and therefore transmissioncost can be reduced.

A seventh aspect of the present invention is directed to a headercompression method of compressing a header of a packet to be transmittedby referring to reference information that is also included in areceiving side, and the method includes: a packet compression step ofcompressing the header of the packet in a specified operation mode byreferring to the reference information stored, and selectively adding,to the compressed packet, update information for updating the referenceinformation at the receiving side; a packet transmission step oftransmitting the packet compressed in the packet compression step; apacket receiving step of receiving an ACK packet indicating that thereference information at the receiving side has been correctly updatedor a NACK packet for requesting transmission of the update informationdue to a header decompression error that occurred at the receiving side;a delay time measuring step of measuring a roundtrip delay time withrespect to the receiving side by transmitting and receiving a packet toand from the receiving side; and a mode determination step of switchingthe operation mode of the packet compression step to a reliable modewhere, after the reference information is updated, the updateinformation is continuously added until the ACK packet is received, andan optimistic mode where the update information is added when thereference information to be referred to is updated and whenever the NACKpacket is received. In the mode determination step, the roundtrip delaytime measured by a unit time X in the delay time measuring step isprovided, and when the operation mode is the optimistic mode and theroundtrip delay time is smaller than a predetermined value Y, theoperation mode is switched to the reliable mode, and when the operationmode is the reliable mode and the roundtrip delay time is larger than apredetermined value Z, the operation mode is switched to the optimisticmode.

According to the seventh aspect, based on the roundtrip delay time withrespect to the receiving side, the transmitting side switches the headercompression scheme to the reliable mode when the roundtrip delay time isshort, and to the optimistic mode when long. As such, by switching theheader compression scheme by the transmitting side, the headercompression efficiency and the transmission quality during wirelesstransmission can be improved. In other words, it is possible to reducetime and cost required for transmission of a packet that cannot bedecompressed, and also improve compression efficiency and greatly reducecost required for packet transmission.

In this case, in the mode determination step, a rate of change in theroundtrip delay time measured by the unit time X is calculated, and theunit time X is increased when the rate of change is smaller than apredetermined value A, and decreased when the rate of change is largerthan a predetermined value B.

Thus, when determining that the transmission quality is stable since theroundtrip delay time is roughly stable, the transmitting side extendsthe time interval between possible changes of the header compressionscheme and, otherwise, shortens the time internal. As such, highlyresponsive to the change in transmission quality during wirelesstransmission, the transmitting side switches the header compressionscheme, thereby improving the compression efficiency and thetransmission quality during wireless transmission. In other words,header decompression errors are suppressed for high compressionefficiency, and therefore transmission cost can be reduced.

An eighth aspect of the present invention is directed to a headerdecompression method of decompressing a header of a received packet byreferring to reference information that is also included in atransmitting side, and the method includes: a packet receiving step ofreceiving the packet with update information selectively added theretofor updating the reference information stored; a packet decompressionstep, provided with the packet received in the packet receiving step, ofupdating the update information by using the update information, anddecompressing the header of the packet by referring to the referenceinformation; a packet transmission step of transmitting an ACK packetindicating that the reference information has been correctly updated ora NACK packet for requesting transmission of the update information whena header decompression error occurs in the packet decompression step; adelay time measuring step of measuring a roundtrip delay time withrespect to the transmitting side by transmitting and receiving a packetto and from the transmitting side; a mode determination step ofswitching the operation mode of the transmitting side to a reliable modewhere, after updating the reference information of the transmittingside, the transmitting side continuously adds the update informationuntil receiving the ACK packet, and to an optimistic mode where thetransmitting side adds the update information when the referenceinformation of the transmitting side is updated and whenever receivingthe NACK packet; and a mode notification step of notifying thetransmitting side of the operation mode selected in the modedetermination step. In the mode determination step, the roundtrip delaytime measured by a unit time X in the delay time measuring step, and theoperation mode is switched to the reliable mode when the operation modeis the optimistic mode and the roundtrip delay time is smaller than apredetermined value Y, and to the optimistic mode when the operationmode is the reliable mode and the roundtrip delay time is larger than apredetermined value Z.

According to the eighth aspect, based on the roundtrip delay time withrespect to the transmitting side, the receiving side switches the headercompression scheme to the reliable mode when the roundtrip delay time isshort, and to the optimistic mode when long. As such, by switching theheader compression scheme by the receiving side, the header compressionefficiency and the transmission quality during wireless transmission canbe improved. In other words, it is possible to reduce time and costrequired for transmission of a packet that cannot be decompressed, andalso improve compression efficiency and greatly reduce cost required forpacket transmission.

In this case, in the mode determination step, a rate of change in theroundtrip delay time measured by the unit time X is calculated, and theunit time X is increased when the rate of change is smaller than apredetermined value A, and decreased when the rate of change is largerthan a predetermined value B.

Thus, when determining that the transmission quality is stable since theroundtrip delay time is roughly stable, the receiving side extends thetime interval between possible changes of the header compression schemeand, otherwise, shortens the time internal. As such, highly responsiveto the change in transmission quality during wireless transmission, thereceiving side switches the header compression scheme of thetransmitting side, thereby improving the compression efficiency and thetransmission quality during wireless transmission. In other words,header decompression errors are suppressed for high compressionefficiency, and therefore transmission cost can be reduced.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the structure of a data transmittingapparatus and a data receiving apparatus according to a first embodimentof the present invention;

FIG. 2 is a diagram for demonstrating three-step handshake carried outwhen a header compression scheme is switched;

FIG. 3 is a block diagram showing the structure of a data transmittingapparatus and a data receiving apparatus according to a secondembodiment of the present invention;

FIG. 4 is a block diagram showing the structure of a data transmittingapparatus and a data receiving apparatus according to a third embodimentof the present invention;

FIG. 5 is a block diagram showing the structure of a data transmittingapparatus and a data receiving apparatus according to a fourthembodiment of the present invention;

FIG. 6A is a diagram showing a communication network not includingwireless communication;

FIG. 6B is a diagram showing a communication network including wirelesscommunication;

FIG. 7 is a block diagram showing the structure of a conventional datatransmitting apparatus and a conventional data receiving apparatus usingROHC;

FIG. 8 is a sequence chart of ROHC;

FIG. 9 is a sequence chart of a reliable mode;

FIG. 10 is a normal sequence chart of an optimistic mode; and

FIG. 11 is an abnormal sequence chart of the optimistic mode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Described below are a data transmitting apparatus and a data receivingapparatus according to each embodiment of the present invention. Theobject of the present invention is to improve compression efficiency andtransmission quality during wireless transmission by dynamicallyswitching a header compression scheme depending on the quality of atransmission path or the state of a roundtrip delay time. The datatransmitting apparatus according to each embodiment of the presentinvention transmits each packet after compressing the header thereof. Inthis sense, the data transmitting apparatus can be regarded as a datacompression apparatus or a header compression apparatus. Also, the datareceiving apparatus according to each embodiment of the presentinvention decompresses the header of each received packet. In thissense, the data receiving apparatus can be regarded as a datadecompression apparatus or a header decompression apparatus. Note thatdescribed below is a case of unidirectional communication from the datatransmitting apparatus to the data receiving apparatus, but thedescription can also be applied to bi-directional communication carriedout by two apparatuses each having transmission and receivingcapabilities and each connected to a network.

First Embodiment

FIG. 1 is a data transmitting apparatus and a data receiving apparatusaccording to a first embodiment of the present invention. In FIG. 1, adata transmitting apparatus 1 includes a packet input unit 11, a packetcompressor 12, a packet transmitter 13, an ACK/NACK packet receiver 14,a reference information manager 15, and a mode determination unit 31. Adata receiving apparatus 2 includes a packet receiver 21, a packetdecompressor 22, a packet output unit 23, an ACK/NACK packet transmitter24, and a reference information manager 25.

The reference information manager 15 and the reference informationmanager 25 each store and manage the same reference information. Here,the reference information is information indicating a change in eachfield included in the header of the packet from the one included in theheader of the previous packet. The reference information stored in thereference information manager 15 is referred to for compressing theheader of the packet. The reference information stored in the referenceinformation manager 25 is referred to for decompressing the header ofthe received packet.

The packet input unit 11 receives data by unit of a packet, and suppliesit to the packet compressor 12. The packet compressor 12 refers to thereference information stored in the reference information manager 15 tocompress the header of the supplied packet. The mode determination unit31 outputs a mode switching signal 51. By this switching signal 51, theheader compression scheme taken by the packet compressor 12 is switchedto either one of the reliable mode or the optimistic mode as describedin Background Art section. The packet transmitter 13 transmits thepacket compressed by the packet compressor 12 to the data receivingapparatus 2.

When the reliable mode is taken, the packet compressor 12 updates thereference information stored in the reference information manager 15,and then adds update information to each packet until receiving anotification of receiving an ACK packet from the ACK/NACK packetreceiver 14. Here, the ACK packet is a packet indicating that thereference information in the data receiving apparatus 2 has beencorrectly updated, and the update information is information forupdating the reference information at the receiving side. Also, thepacket compressor 12 refers to the updated reference information tocompress the header of the packet with the update information addedthereto and the packets coming thereafter.

When the optimistic mode is taken, the packet compressor 12 updates thereference information stored in the reference information manager 15,then adds the update information to only one packet and, thereafter,outputs packets without adding the update information thereto. Then, ifreceiving a notification of receiving a NACK packet from the ACK/NACKpacket receiver 14, the packet compressor 12 adds the update informationto the next packet. Here, the NACK packet is a packet for requestingtransmission of the update information, the packet being transmittedwhen the data receiving apparatus 2 detects a header decompressionerror. The packet compressor 12 refers to the updated referenceinformation to compress the header of the packet with the updateinformation added thereto and the headers of the packets comingthereafter.

The packet compressor 12 takes an arbitrary scheme to know when thereference information should be updated. By way of example, the packetcompressor 12 may analyze the packet to determine whether the referenceinformation should be updated or not. Alternatively, update of thereference information may be specified when the packet is inputted tothe packet input unit 11.

The packet receiver 21 receives the packet transmitted from the datatransmitting apparatus 1, and supplies it to the packet decompressor 22.The packet decompressor 22 decompresses the header of the receivedpacket by referring to the reference information stored in the referenceinformation manager 25, and outputs the packet to the packet output unit23. The packet output unit 23 outputs the packet including thedecompressed header.

When receiving the packet with the update information added thereto, thepacket decompressor 22 updates the reference information stored in thereference information manager 25, and notifies the ACK/NACK packettransmitter 24 of the update. After decompression, the packetdecompressor 22 carries out CRC described in the above document(“draft-ietf-rohc-rtp-00.txt 29 Jun. 2000”) , for example, to checkwhether or not the header has been correctly decompressed. Whendetecting any decompression error in the header, the packet decompressor22 notifies the ACK/NACK packet transmitter 24 of the error.

The ACK/NACK packet transmitter 24 transmits, based on the notificationfrom the packet decompressor 22, either one of the ACK packet indicatingthat the reference information has been correctly updated or the NACKpacket requesting transmission of the update information, to the datatransmitting apparatus 1.

The ACK/NACK packet receiver 14 receives the ACK packet or the NACKpacket transmitted from the ACK/NACK packet receiver 14, and outputs theACK or NACK packet to the packet compressor 12 and the modedetermination unit 31.

The mode determination unit 31 determines, based on the ACK packet orthe NACK packet received by the ACK/NACK packet receiver 14, which ofthe reliable mode and the optimistic mode should be taken in the packetcompressor 12.

When the data transmitting apparatus 1 and the data receiving apparatus2 are both in the optimistic mode, the mode determination unit 31 countsthe number of NACK packets received by a unit time X by the ACK/NACKpacket receiver 14. The number of NACK packets corresponds to the numberof header decompression errors that occurred in the data receivingapparatus 2. When the number of NACK packets received by the unit time Xis larger than a predetermined value Y, the mode determination unit 31determines that the transmission quality is degraded, and outputs themode switching signal 51 for instruction of switching to the reliablemode.

On the other hand, when the data transmitting apparatus 1 and the datareceiving apparatus 2 are both in the reliable mode, the modedetermination unit 31 counts the number of ACK packets received by theunit time X by the ACK/NACK packet receiver 14. The number of ACKpackets corresponds to the number of reference information updatescorrectly carried out in the data receiving apparatus 2. When the numberof ACK packets received by the unit time X is larger than apredetermined value Z, the mode determination unit 31 determines thatthe transmission quality is improved, and outputs the mode switchingsignal 51 for instruction of switching to the optimistic mode.

The packet compressor 12 switches the header compression scheme betweenthe reliable mode and the optimistic mode based on the mode switchingsignal 51 outputted from the mode determination unit 31. For thisswitching, the packet compressor 12 carries out three-step handshake asdescribed in detail below to confirm the switching of the headercompression scheme in the packet decompressor 22, and then switches theheader compression scheme to be taken by itself. In the first step ofthe three-step handshake, switching instruction information istransmitted from the data transmitting apparatus 1 to the data receivingapparatus 2. In the second step, a packet for confirming the switchingof the header compression scheme is transmitted from the data receivingapparatus 2 to the data transmitting apparatus 1. In the third step, apacket compressed by the switched header compression scheme istransmitted from the data transmitting apparatus 1 to the data receivingapparatus 2.

FIG. 2 is a diagram for exemplarily demonstrating the three-stephandshake carried out for switching the header compression scheme. Inthis example, the header compression scheme is switched from theoptimistic mode to the reliable mode, in both the data transmittingapparatus 1 and the data receiving apparatus 2. Before switching, eachpacket has been transmitted in the optimistic mode. When determiningthat the header compression scheme should be switched, the datatransmitting apparatus 1 transmits a switching instruction information61 for switching the header compression scheme. The data transmittingapparatus 1 may transmit this switching instruction information 61 byadding it to the packet containing data, or by using a control packetnot containing any data. After transmitting the switching instructioninformation 61, the data transmitting apparatus 1 waits until receivinga switching-confirmed packet 62. On receiving the switching instructioninformation 61, the data receiving apparatus 2 switches its own headercompression scheme, and also transmits the switching-confirmed packet 62to the data transmitting apparatus 1. After receiving theswitching-confirmed packet 62, the data transmitting apparatus 1 takesthe switched header compression scheme, that is, operates in thereliable mode, for packet transmission.

The three-step handshake is described in detail in the above document(“draft-ietf-rohc-rtp-00.txt 29 Jun. 2000”). Alternatively, any othermethod may be taken for switching the header compression scheme in thedata transmitting apparatus 1 and the data receiving apparatus 2.

As stated above, according to the present embodiment, the transmittingside determines the transmission quality based on the state of updatingthe reference information in the receiving side, and switches the headercompression scheme to the reliable mode when the transmission quality isdegraded and to the optimistic mode when improved. As such, by switchingthe header compression scheme in the transmitting side, headercompression efficiency and transmission quality during wirelesscommunication can be improved.

Example Modification of First Embodiment

In the first embodiment, the unit time X referred to by the modedetermination unit 31 does not have any limitation. Therefore, the valueof the unit time X may be dynamically varied depending on the number ofACK packets or NACK packets received.

The mode determination unit according to one example modification of thefirst embodiment counts the number of ACK packets or NACK packetsreceived, and also divides the counted number by the number previouslycounted, thereby calculating the rate of change in the number ofreceived packets. When the calculated rate of change is smaller than apredetermined value A, the mode determination unit determines that thetransmission quality is stable, and increases the value of the unit timeX. On the other hand, when the calculated rate of change is larger thana predetermined value B, the mode determination unit determines that thetransmission quality is frequently varied, and decreases the value ofthe unit time X. Note that the rate of change in the number of packetsis not restricted to the value calculated by division as describedabove, but may be a value calculated based on history of the number ofreceived packets stored.

As such, in the present example modification, the transmitting sideextends the time interval between possible changes in the headercompression scheme when determining that the transmission quality isstable and, otherwise, shortens the time interval. As such, highlyresponsive to the change in transmission quality during wirelesstransmission, the transmitting side switches the header compressionscheme, thereby improving the compression efficiency and thetransmission quality during wireless transmission.

Second Embodiment

FIG. 3 is a block diagram showing the structure of a data transmittingapparatus and a data receiving apparatus according to a secondembodiment of the present invention. In FIG. 3, a data transmittingapparatus 3 includes a mode change request receiver 41, and a datareceiving apparatus 4 includes a mode determination unit 32 and a modechange request transmitter 42. In the second embodiment, the datareceiving apparatus 4 determines the switching of the header compressionscheme, which is different from the first embodiment where the datatransmitting apparatus 1 determines the switching. In the secondembodiment, components that are the same as those in the firstembodiment are provided with the same reference numerals, and notdescribed herein.

When detecting any header decompression error, the packet decompressor22 also notifies the mode determination unit 32 of the error. The modedetermination unit 32 counts the number of header decompression errorsthat occurred by the unit time X in the packet decompressor 22, and thenoperates as follows.

When the data transmitting apparatus 3 and the data receiving apparatus4 are both in the optimistic mode, if the counted number of headerdecompression errors is larger than the predetermined value Y, the modedetermination unit 32 determines that the transmission quality isdegraded, and outputs a mode switching signal 52 a for switching to thereliable mode.

On the other hand, the data transmitting apparatus 3 and the datareceiving apparatus 4 are both in the reliable mode, if the countednumber of header decompression errors is smaller than the predeterminedvalue Z, the mode determination unit 32 determines that the transmissionquality is improved, and outputs the mode switching signal 52 a forswitching to the optimistic mode.

When the mode determination unit 31 outputs the mode switching signal 52a, the mode change request transmitter 42 transmits a packet containingthe information about the mode switching signal 52 a. The mode changerequest receiver 41 receives the packet transmitted from the mode changerequest transmitter 42, and outputs a mode switching signal 52 b havingthe same value as that of the mode switching signal 52 a to the packetcompressor 12.

The packet compressor 12 switches the header compression scheme based onthe mode switching signal 52 b outputted from the mode change requestreceiver 41. For this switching, as with the first embodiment, thethree-step handshake is carried out, for example.

As stated above, according to the present embodiment, the receiving sidedetermines the transmission quality based on the state of occurrence ofheader decompression errors. If determining that the transmissionquality is degraded, the header compression scheme at the transmittingside is switched to the reliable mode and, if improved, to theoptimistic mode. As such, the header compression scheme at thetransmitting side is switched by the receiving side, thereby improvingthe header compression efficiency and the transmission quality duringwireless communication.

Third Embodiment

FIG. 4 is a block diagram showing a data transmitting apparatus and adata receiving apparatus according to a third embodiment of the presentinvention. In FIG. 4, a data transmitting apparatus 5 includes a modedetermination unit 33 and a delay time measuring unit 43 for measuring aroundtrip delay time, and a data receiving apparatus 6 includes aresponding unit 44 for responding to the measurement of the roundtripdelay time. In the third embodiment, the switching of the headercompression scheme is determined at the transmitting side based on theroundtrip delay time, which is different from the first embodiment wherethe switching is determined based on the number of received ACK packetsor NACK packets. In the third embodiment, components that are the sameas those in the first embodiment are provided with the same referencenumerals, and not described herein.

To measure a roundtrip delay time between the data transmittingapparatus 1 and the data receiving apparatus 2, the delay time measuringunit 43 transmits, at predetermined time intervals X, a delay timemeasuring packet with a timestamp added thereto. The responding unit 44receives the delay time measuring packet, and then returns it to thedelay time measuring unit 43. On receiving the returned delay timemeasuring packet, the delay time measuring unit 43 calculates thedifference between a receiving time and a timestamp added to the packetfor finding the roundtrip delay time between the data transmittingapparatus 5 and the data receiving apparatus 2, and outputs thecalculation result to the mode determination unit 33. By way of example,the delay time measuring packet may be implemented by the RTCP (RealTime Control Protocol) packet described in RFC 1889. How to measure thedelay time is not restricted to the above, and any other method may betaken.

Based on the received roundtrip delay time, the mode determination unit33 operates as follows. When the data transmitting apparatus 5 and thedata receiving apparatus 6 are both in the optimistic mode, if the valueof the roundtrip delay time is smaller than the predetermined time Y,the mode determination unit 33 determines that the transmissionreliability should be increased since it does not take much time toreceive the ACK packet from the data receiving apparatus 6. Therefore,the mode determination unit 33 outputs a mode switching signal 53 forswitching to the reliable mode where the occurrence of headerdecompression error is suppressed and the transmission is reliable more.

On the other hand, when the data transmitting apparatus 5 and the datareceiving apparatus 6 are both in the reliable mode, if the value of theroundtrip delay time is larger than the predetermined time Z, the modedetermination unit 33 determines that degradation in compressionefficiency should be prevented since it takes much time to receive theACK packet from the data receiving apparatus 6. Therefore, the modedetermination unit 33 outputs the mode switching signal 53 for switchingto the optimistic mode.

The packet compression unit 12 switches the header compression schemebased on the mode switching signal 53 outputted from the modedetermination unit 33. For this switching, as with the first embodiment,the three-step handshake is carried out, for example.

As stated above, according to the present embodiment, based on theroundtrip delay time between the transmitting side and the receivingside, the transmitting side switches the header compression scheme tothe reliable mode when the roundtrip delay time is short, and to theoptimistic mode when long. As such, by switching the header compressionscheme at the transmitting side, the header compression efficiency andthe transmission quality can be improved during wireless communication.

Fourth Embodiment

FIG. 5 is a block diagram showing a data transmitting apparatus and adata receiving apparatus according to a fourth embodiment of the presentinvention. A data transmitting apparatus 7 includes the mode changerequest receiver 41 and the responding unit 44, and a data receivingapparatus 8 includes a mode determination unit 34, the mode changerequest transmitter 42, and the delay time measuring unit 43. In thefourth embodiment, the switching of the header compression scheme isdetermined at the receiving side based on the roundtrip delay time,which is different from the second embodiment where the switching isdetermined based on the number of received ACK packets or NACK packets.In the fourth embodiment, components that are the same as those in thefirst to third embodiments are provided with the same referencenumerals. The detail of the present embodiment is evident from the abovedescription of the first to third embodiments, and therefore notdescribed herein.

According to the fourth embodiment, based on the roundtrip delay timebetween the transmitting side and the receiving side, the receiving sideswitches the header compression scheme to the reliable mode if the delaytime is short, and to the optimistic mode if long. As such, by switchingthe header compression scheme at the receiving side, the headercompression efficiency and the transmission quality can be improvedduring wireless communication.

Here, example modification is mentioned only for the first embodiment.However, similar example modification can be applied also to the secondto fourth embodiments. That is, as to the value of the number of headerdecompression errors in the data receiving apparatus or the value of theroundtrip delay time, the mode determination unit increases the unittime X when the rate of change in the value is smaller than thepredetermined value A, and decreases the unit time X when the rate ofchange is larger than the predetermined value B. In any examplemodification, highly responsive to the change in transmission qualityduring wireless transmission, the transmitting or receiving sideswitches the header compression scheme, thereby improving thecompression efficiency and the transmission quality during wirelesstransmission.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

1. A header compression apparatus for compressing a header of a packetto be transmitted by referring to reference information that is alsoincluded in a receiving side, said header compression apparatuscomprising: a reference information manager operable to store and managethe reference information; a packet compressor operable to compress theheader of the packet in a specified operation mode by referring to thereference information, and to selectively add, to the compressed packet,update information for updating the reference information at thereceiving side; a packet transmitter operable to transmit the packetcompressed by said packet compressor; a packet receiver operable toreceive an ACK packet indicating that the reference information at thereceiving side has been correctly updated or a NACK packet forrequesting transmission of the update information due to a headerdecompression error that occurred at the receiving side; and a modeswitching unit operable to dynamically switch, based on one of a numberof ACK packets received and a number of NACK packets received, theoperation mode of said packet compressor between a reliable mode where,after the reference information of said header compression apparatus isupdated, said packet compressor continuously adds the update informationuntil the ACK packet is received, and an optimistic mode where saidpacket compressor adds the update information when the referenceinformation of said header compression apparatus is updated and wheneverreceiving the NACK packet.
 2. A header decompression apparatus fordecompressing a header of a received packet by referring to referenceinformation that is also included in a transmitting side, said headerdecompression apparatus comprising: a reference information manageroperable to store and manage the reference information; a packetreceiver operable to receive the packet with update informationselectively added thereto for updating the reference information; apacket decompressor, provided with the packet received by said packetreceiver, operable to update the reference information by using theupdate information, and to decompress the header of the packet byreferring to the reference information; a packet transmitter operable totransmit an ACK packet indicating that the reference information hasbeen correctly updated or a NACK packet for requesting transmission ofthe update information when a header decompression error occurs in saidpacket decompressor; a mode switching unit operable to dynamicallyswitch, based on one of a number of ACK packets received and a number ofNACK packets received, an operation mode of the transmitting sidebetween a reliable mode where, after updating the reference informationof the transmitting side, the transmitting side continuously adds theupdate information until receiving the ACK packet, and an optimisticmode where the transmitting side adds the update information when thereference information of the transmitting side is updated and wheneverreceiving the NACK packet; and a mode notification unit operable tonotify the transmitting side of the operation mode selected by said modeswitching unit.
 3. A header compression apparatus for compressing aheader of a packet to be transmitted by referring to referenceinformation that is also included in a receiving side, said headercompression apparatus comprising: a reference information manageroperable to store and manage the reference information; a packetcompressor operable to compress the header of the packet in a specifiedoperation mode by referring to the reference information, and toselectively add, to the compressed packet, update information forupdating the reference information at the receiving side; a packettransmitter operable to transmit the packet compressed by said packetcompressor; a packet receiver operable to receive an ACK packetindicating that the reference information at the receiving side has beencorrectly updated or a NACK packet for requesting transmission of theupdate information due to a header decompression error that occurred atthe receiving side; a delay time measuring unit operable to measure aroundtrip delay time with respect to the receiving side by transmittingand receiving a packet to and from the receiving side; and a modeswitching unit operable to dynamically switch, based on the roundtripdelay time, the operation mode of said packet compressor between areliable mode where, after the reference information of said headercompression apparatus is updated, said packet compressor continuouslyadds the update information until the ACK packet is received, and anoptimistic mode where said packet compressor adds the update informationwhen the reference information of said header compression apparatus isupdated and whenever receiving the NACK packet.
 4. A headerdecompression apparatus for decompressing a header of a received packetby referring to reference information that is also included in atransmitting side, said header decompression apparatus comprising: areference information manager operable to store and manage the referenceinformation; a packet receiver operable to receive the packet withupdate information selectively added thereto for updating the referenceinformation; a packet decompressor, provided with the packet received bysaid packet receiver, operable to update the reference information byusing the update information, and to decompress the header of the packetby referring to the reference information; a packet transmitter operableto transmit an ACK packet indicating that the reference information hasbeen correctly updated or a NACK packet for requesting transmission ofthe update information when a header decompression error occurs in saidpacket decompressor; a delay time measuring unit operable to measure aroundtrip delay time with respect to the transmitting side bytransmitting and receiving a packet to and from the transmitting side; amode switching unit operable to dynamically switch, based on theroundtrip delay time, an operation mode of the transmitting side betweena reliable mode where, after updating the reference information of thetransmitting side, the transmitting side continuously adds the updateinformation until receiving the ACK packet, and an optimistic mode wherethe transmitting side adds the update information when the referenceinformation of the transmitting side is updated and whenever receivingthe NACK packet; and a mode notification unit operable to notify thetransmitting side of the operation mode selected by said mode switchingunit.
 5. A header compression method of compressing a header of a packetto be transmitted by referring to reference information that is alsoincluded in a receiving side, said header compression method comprising:compressing a packet by compressing the header of the packet in aspecified operation mode by referring to the reference informationstored, and selectively adding, to the compressed packet, updateinformation for updating the reference information at the receivingside; transmitting the packet compressed in said compressing; receivingan ACK packet indicating that the reference information at the receivingside has been correctly updated or a NACK packet for requestingtransmission of the update information due to a header decompressionerror that occurred at the receiving side; and dynamically switching,based on one of a number of ACK packets received and a number of NACKpackets received, the operation mode of said compressing between areliable mode where, after the reference information is updated, theupdate information is continuously added until the ACK packet isreceived, and an optimistic mode where the update information is addedwhen the reference information is updated and whenever the NACK packetis received.
 6. A header decompression method of decompressing a headerof a received packet by referring to reference information that is alsoincluded in a transmitting side, said header decompression methodcomprising: receiving the packet with update information selectivelyadded thereto for updating the reference information stored;decompressing the packet received during said receiving, by updating thereference information by using the update information, and decompressingthe header of the packet by referring to the reference information;transmitting an ACK packet indicating that the reference information hasbeen correctly updated or a NACK packet for requesting transmission ofthe update information when a header decompression error occurs in saiddecompressing; dynamically switching, based on one of a number of ACKpackets received and a number of NACK packets received, an operationmode of the transmitting side between a reliable mode where, afterupdating the reference information of the transmitting side, thetransmitting side continuously adds the update information untilreceiving the ACK packet, and an optimistic mode where the transmittingside adds the update information when the reference information of thetransmitting side is updated and whenever receiving the NACK packet; andnotifying the transmitting side of the operation mode selected in saidswitching.
 7. A header compression method of compressing a header of apacket to be transmitted by referring to reference information that isalso included in a receiving side, said header compression methodcomprising: compressing a packet by compressing the header of the packetin a specified operation mode by referring to the reference informationstored, and selectively adding, to the compressed packet, updateinformation for updating the reference information at the receivingside; transmitting the packet compressed in said compressing; receivingan ACK packet indicating that the reference information at the receivingside has been correctly updated or a NACK packet for requestingtransmission of the update information due to a header decompressionerror that occurred at the receiving side; measuring a roundtrip delaytime with respect to the receiving side by transmitting and receiving apacket to and from the receiving side; and dynamically switching, basedon the roundtrip delay time, the operation mode of said compressingbetween a reliable mode where, after the reference information to bereferred to is updated, the update information is continuously addeduntil the ACK packet is received, and an optimistic mode where theupdate information is added when the reference information is updatedand whenever the NACK packet is received.
 8. A header decompressionmethod of decompressing a header of a received packet by referring toreference information that is also included in a transmitting side, saidheader decompression method comprising: receiving the packet with updateinformation selectively added thereto for updating the referenceinformation stored; decompressing the packet received during saidreceiving, by updating the reference information by using the updateinformation, and decompressing the header of the packet by referring tothe reference information; transmitting an ACK packet indicating thatthe reference information has been correctly updated or a NACK packetfor requesting transmission of the update information when a headerdecompression error occurs in said decompressing; measuring a roundtripdelay time with respect to the transmitting side by transmitting andreceiving a packet to and from the transmitting side; dynamicallyswitching, based on the roundtrip delay time, an operation mode of thetransmitting side between a reliable mode where, after updating thereference information of the transmitting side, the transmitting sidecontinuously adds the update information until receiving the ACK packet,ands an optimistic mode where the transmitting side adds the updateinformation when the reference information of the transmitting side isupdated and whenever receiving the NACK packet; and notifying thetransmitting side of the operation mode selected in said switching.